Novel process and composition for brightening aluminum



United States Patent 3,410,803 NOVEL PROCESS AND COMPOSITION FOR BRIGHTENIN G ALUMINUM Scott Hoover, Park Forest, 111., assignor to Stauffer Chemical Company, New York, N.Y., a corporation of Delaware No Drawing. Filed Nov. 30, '1965, Ser. No. 510,665

8 Claims. (Cl. 25279.2)

ABSTRACT OF THE DISCLOSURE Process for brightening aluminum and aluminum alloys having less than about 8% zinc, 4% copper and 1% silicon which comprises immersing the said aluminum and aluminum alloys in a bath consisting essentially of 5-50 p.p.m. silver, 15% nitric acid, 28% water and 6083% orthophosphoric acid.

This invention relates to metal treating and in particular to methods and compositions for brightening aluminum and aluminum alloys.

Chemical brightening, hereinafter referred to as brightd ipping, is a method for brightening and smoothing aluminum products by making use of the solution potential of the aluminum surface in a bath usually including mixtures of sulfuric, hydrochloric, nitric, phosphoric or acetic acids. Of the various types of concentrated baths, the phosphoric-nitric bath has found almost universal acceptance in the United States. Despite its general acceptance, the phosphoric-nitric bath is particularly difficult to control, partly because of the rather narrow range of temperature and component concentration permitted for optimum brightening. In addition, the high temperatures required cause the volatile bath components, such as the Water and nitric acid, to evaporate rapidly. Thus one must constantly replenish the bath in order to avoid preferential dissolution of the aluminum alloys. The latter effect is responsible for the surface irregularities known as etching, pitting, and orange peel. In order to avoid these problems, it has been common to use more water than desirable for optimum brightening.

Another problem has been the difficulty of removing surface markings already on the metal such as die marks, millmarks, and minor scratches. Such surface irregularities have not heretofore been appreciably affected by the bright-dip bath.

Heretofore workers have sought to alleviate or reduce the above mentioned problems by the use of various additives. For example, US. Patent No. 2,593,449 teaches the use of a metal salt, preferably a copper salt, in a bath composition comprising .5 to 5% nitric acid, .01 to .5 hydrofluoric acid, about .05 to 1% orthophosphoric acid, and from about 0.1 to 0.5% ammonia by weight of the solution. For use in a brightening solution consisting essentially of fluoride ion and nitric acid, US. Patent 2,719,781 teaches the incorporation of various metals, preferably copper ions. A further example is US. Patent 2,967,136 wherein a bath composition comprising essentially sulfuric acid and phosphoric acid contains a silver salt. Despite the use of various metal salts in bright dipping compositions, the use of these various metal salts, particularly silver salts, has not proved beneficial heretofore to applicants knowledge, in a bath composition comprising essentially phosphoric acid and nitric acid. This failure to appreciate the advantages obtainable by incorporation of a minor amount of silver ion into the most commonly used bath in the United States, i.e., the phosphoric-nitric acid bath, is probably predominantly the result of using too large a silver ion concentration, maintaining the bath at less than ideal temperatures and addi- 3,410,803 Patented Nov. 12, 1968 tionally often including too great a sulfuric acid concentration. Those familiar with this art are cognizant that ohtaining a bright-dipped aluminum or aluminum alloy having high specular reflectance is not a simple matter and is largely a matter of empirical considerations.

It is accordingly an object of this invention to provide a bright dipping composition comprising mainly phosphoric acid, nitric acid, and water, which composition is effective in brightening aluminum and aluminum alloys over a wide range of operating conditions; thus enabling one to broaden the rather narrow conditions presently required for acceptable specular reflectance.

Another object is to provide a bright dipping composition comprising mainly phosphoric acid, nitric acid, aluminum phosphate and water, which composition is able to produce a high specular reflectance Without requiring after-treatment of the aluminum workpiece.

A further object is to provide a bright-dipping composition comprising mainly phosphoric acid, nitric acid, aluminum phosphate and Water, which composition is effective in brightening certain problem aluminum alloys and which is able to produce a high specular reflectance despite the presence of various by-products heretofore found harmful.

A still further object is to provide a process for chemically polishing aluminum and aluminum alloys in a bath comprising mainly phosphoric acid, nitric acid, aluminum phosphate and water, which process is effective in reducing the undesirable appearance of die marks, millmarks and scratches on the metal to be bright-dipped and the corrosive effect of the bath composition.

An additional object is to provide a process for brightening aluminum and aluminum alloys which may be operated at lower temperatures than heretofore and which consequently is effective in reducing the evaporation of the bath components and also the corrosive action of said components.

Other objects will be apparent from the detailed explanation which follows:

In accordance with the present invention, it has been discovered that many of the prior art problems can be overcome or greatly reduced and other benefits achieved by incorporating a very small and critical amount of silver ion into the phosphoric acid-nitric acid bath. This very small amount of silver enables one to broaden the operating conditions by more than doubling the water range previously permitted in the bath and conversely decrease proportionally the concentration of phosphoric acid required, while still obtaining a specular reflectance acceptable for commercial production. In addition, one is able to obtain a greater specular reflectance on the metal than heretofore possible when operating within the narrow range previously required. Another benefit of the invention is that the bath may be operated at temperatures from 10 to 20 F. lower than heretofore with no loss in brightness. This allows one a broader operational temperature range. Moreover, the addition of silver ion retards the attack on the metal bright dipping tank by the phosphoric acid and also inhibits aluminum phosphate crystallization at the surface of the bath. The requirement for multiple baths has been obviated, since a high quality performance may be obtained on a wide variety of aluminum alloys by the use of a bath containing a small amount of silver.

Although it has been generally known heretofore to add various metal ions (including silver) to bright dipping baths to improve specular reflectivity, the effect of the very minor amounts of silver ion disclosed and claimed hereinafter in a phosphoric acid-nitric acid bath is highly unexpected. Surprisingly, we have found that one of the disadvantages prevalent heretofore, namely plating of the silver ion on the aluminum workpiece during bright dipping, can be avoided by maintaining the silver ion concentration of the bath within the critical range stated hereinafter. The plating out of the silver ion on the aluminum workpiece can be a serious disadvantage since it requires postrinsing of the aluminum (usually in a nitric acid bath) to remove the silver before a bright anodized surface can be produced. Nor has it been known hereto-- fore that the composition of the bath may be varied widely Without decreasing the specular reflectance by incorporating in the bath the very minor amounts of silver ion disclosed herein.

Any of a number of silver salts may be used to supply the silver ion including silver sulphate, silver nitrate, silver phosphate, silver carbonate and silver fluoride. The preferred salt is silver nitrate because of its availability and economic advantage. Silver chloride is not suitable because it is too insoluble and precipitates out of solution. Consequently, where the bath contains small amounts of chloride ion as an impurity, it will be necessary to compensate for this by adding more silver. The bath components preferably should be reasonably free of chloride ion and in this connection the use of de-ionized water and substantially pure phosphoric acid is preferred in order to avoid silver precipitation with contaminating chloride ion.

The silver ion concentration contemplated by the invention should be between about 5 and 50 p.p.m., although under certain specific circumstances an acceptable bath may contain up to 200 ppm. silver ion. The preferred concentration is between about 5 and about 30 p.p.m. It will normally not be desirable to have more than 50 ppm. silver. As mentioned supra, any excess silver tends to plate on the aluminum. Consequently, the preferred concentration of silver contemplated by the invention enables one to anodize the aluminum without the necessity of rinsing in an acid bath to remove any silver plating.

In accounting for the superior operation and properties of the bath compositions herein, it is believed that the minor amount of silver ion acts as a catalyst by reason of its eflect in increasing specular reflectance and reducing corrosive action. Its unexpected effect on increasing the phosphoric acid-water operating range is not fully understood.

The bath temperature should be maintained between about 170 F. and 220 F. Pyrophosphoric acid and polyphosphoric acid may be substituted for orthophosphoric acid in the bath but the latter is preferred. We have found that acceptable bright-dipping baths including silver ion in the concentrations stated supra may contain the following components in the concentration ranges enumerated:

Percent H PO 60-83 HNO 1-5 AlPO -1 4 H SO 0-1 2 H O 15-28 The bath may also contain a minor amount (e.g. 100-200 ppm.) of surfactant, such as the nonionic organic surfactants, and a defuming agent such as urea or acetamide ammonium phosphate.

Aluminum and aluminum alloys which may be successfully bright-dipped according to the invention include the 1000 series (commercially pure), the 3000 series (the manganese alloys), the 5000 series (the magnesium alloys), and the 6000 series (the magnesium silicon alloys). Alloy-s which are not brightened successfully are the aluminum-maganese-zinc and aluminum-copper-manganese alloys containing more than 8% zinc and 4% copper respectively and cast aluminum alloys high in silicon (i.e. more than about 1.0 percent).

In the following examples, which will serve to illustrate the invention but should not be construed as limiting the invention since variations and modifications within the true scope thereof will be obvious to one skilled in the art, all percentages are by weig-ht.

To measure the eflicacy of a minor amount of silver ion on broadening bath operating conditions, the following test was conducted. Bath solutions were prepared containing the following components:

Percent AlPO 10 .9 H PO 68.5 H O 18.0 HNO 2.6

The baths were maintained at a temperature of 200 F. with a specific gravity of 61.6 Baum. Sufficient silver nitrate was added to some of these baths to provide 30 and 50 ppm. free silver ion respectively.

Example 1 2" x 5" Alcoa No. 5457 (0.8- /2% Mg, 0.8% Si, Fe, .20% Cu, .15-.45% Mn, others .10%, remainder Al) buffed and degreased panels were dipped for two minutes in 1800 ml. of the baths hereinabove prepared, using hand agitation and a 20-second transfer time. The panels were subsequently anodized for 10 minutes at amps/ft. and -22 C. in a sulfuric acid (15%) solution. After sealing for 10 minutes at 205210 F. in an aqueous solution adjusted to a pH=5.75, by the addition of sodium acetate, the specular reflectance was measured on a Hunter Reflectometer Model D40. A refiectometer reading of 160 was selected as the maximum acceptable specular reflectance, with higher numbers representing a duller finish and lower numbers a brighter finish. This was confirmed by visual observation. The following reflectance readings were obtained from aluminum dipped in said baths:

Test: P.p.m. Ag Reflectometer No. In Bath Reading Example 2 Baths containing the following components were prepared:

Percent AIPO 10.5 H PO 66.0 H O 20.9 HNO 2.6

The temperature was maintained at 200 F. and 220 F. respectively; the specific gravity was 59.9 Baum, and the following reflectance readings were obtained from No. 5457 aluminum panels dipped in baths containing the specified silver ion in accordance with Example 1:

To test the effect of silver ion with very little nitric acid, baths were prepared containing the following component percentages:

Percent AlPO 10.8 H PO 68.3 H O 19.9 HNO 1.0

The baths were maintained at a temperature of 200 F. and 220 F. respectively, with a specific gravity of 61.1 Baum and the following reflectance readings were obtained from No. 5457 aluminum panels dipped in baths containing the specified silver ion in accordance with Example 1:

Refiectometer Reflectometer Reading Reading (200 F., bath) (220 F., bath) Example 4 To test the effect of silver ion on baths having a very high Water content, solutions having the following concentrations were prepared:

Percent AlPO 9.6 H P0 60.0 H O 27.8 HNO 2.6

Temperatures were maintained at 200 F. and 220 F. respectively, and the specific gravity was 55.8 Baum.A1uminum panels (No. 5457) dipped into said baths containing the specified silver ion in accordance with Example 1, gave the following reflectance readings:

To test the effect of various organic silver salts 6 baths were prepared containing 2.3% aluminum, 2.6% nitric acid, 20.4% water, and 74.8% phosphoric acid. 1" x 4" buffed aluminum alloy No. 5457 panels were dipped into baths containing the foregoing solution for two minutes while maintaining the temperature at 200 F. To each bath was added enough of the given silver salt to provide a concentration of 50 p.p.m. Ag+ and the reflectivities were read on a Hunter Reflectometer; the higher readings again representing a whiter and more dull finish. The results are shown in the following table where the left column shows the silver additive and the right column the Reflectometer reading:

, Additive: Reflectometer reading None 172 Silver nitrate 136 Benzoic acid silver salt 134 Diphenyl phosphate mono silver salt 129 Silver benzotriazole 142 2,5-dichlor-3,6-dihydroxy-p-benzoquinone disilver salt 141 The procedure for cleaning, bufiing, anodizing and sealing is well documented in the art and thus requires no further mention. See for example Cleaning and Finishing of Nonferrous Metals, pages 611-634, Materials Handbook, vol. 1, 8th edition.

The bright-dipping tank may be lined with any suitable acid resistant material such as karbate, and it is preferred to agitate the bath with air or mechanical agitation.

Having thus described the invention, I claim:

1. A process for brightening aluminum to include aluminum alloys having less than about 8% zinc, 4% copper and 1% silicon which process eliminates the need for post rinsing of the aluminum to remove silver deposits comprising immersing the aluminum in a phosphoric acid-nitric acid bath, reasonably free of chloride ion consisting essentially of:

Percent Silver 5-50 Nitric acid l-5 Water 15-28 Orthophosphoric acid -83 2. The process of claim 1 wherein the silver ion concentration is between about 5 and about 30 parts per million by weight.

3. The process of claim 1 wherein the bath additionally comprises up to 14% aluminum phosphate.

4. The process of claim 1 wherein the bath contains sulfuric acid in an amount not greater than 12%.

5. A composition for brightening aluminum to include aluminum alloys having less than about 8% zinc, 4% copper and 1% silicon which consists essentially of the following ingredients by weight, all of which are reasonably free of chloride ion:

Percent Silver 5-50 Nitric acid 1-5 Water l528 Orthophosphoric acid 60-83 6. The composition of claim 5 wherein the silver ion concentration is from about 5 to about 30 parts per million by weight.

7. The composition of claim 5 additionally containing up to 14% aluminum phosphate.

8. The composition of claim 5 additionally containing sulfuric acid in an amount not greater than 12%.

References Cited UNITED STATES PATENTS 2,650,157 8/1953 Cohran 202-79.2 2,678,875 5/1954 Spooner 202-79.2 XR 2,719,781 10/1955 Hesch 20279.3 XR 2,967,136 l/l96l Cy-briusky et a1. 20414().5 3,194,704 7/1965 Hubert 20279.2 XR

FOREIGN PATENTS 474,228 12/1952 Italy.

MAYER WEINBLAT T, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,410,803 November 12, 1968 Scott Hoover It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, lines 16 and 34, "5-50", each occurrence, should read 5-50 ppm Signed and sealed this 10th day of March 1970.

(SEAL) Attest:

Attesting Officer WILLIAM E. SCHUYLER, JR. 

